Screen tensioning and printing frame

ABSTRACT

A screen tension and printing frame includes sets of parallel rollers coupled at their ends to corner members. Each roller is coupled to an edge portion of a screen. The screen is in sealing contact with a dam member located inwardly of the rollers. Means are provided to camber each of the rollers. Each roller is provided with means to facilitate selective rotation and the locking of the roller in a position so that a desired tension may be applied to the screen.

United States Patent Newman Sept. 30, 1975 [54] SCREEN TENSIONING ANDPRINTING 3.226.861 l/l966 Bird 38/l02.9l FRAME 3 482.343 12/1969 Hamu38/1015 3 60l.9l2 8/l97l Dubbs.... 38/102.9l Inventor: Don Newman,Philadelphia. 3,774.326 11/1973 Selden 38/1014 [73] Assignee: StretchDevices, Inc., Philadelphia,

Prunary Examiner-Geo V. Larkm Attorney. Agent, 0" Firm-Seidel, Gonda &[22] Filed: Sept. 19, 1974 Goldhammer [21] Appl. No.: 507,301

[57] ABSTRACT [52] US. Cl. 38/102.9l A Screen tension and printing frameincludes Sets of [51] Int. Cl. D06C 3/08 parallel rollers coupled theirends to Comer [58] Field of Search 3s/102-102.91; Each is F P an edge ofmull-Ll- 128 128.1. 415.1; 69/191493; screen. The screen is in sealingcontact with a dam 24/2439 24310 2431 1 member located inwardly of therollers. Means are provided to camber each of the rollers. Each rolleris [56] References Cited provided with means to facilitate selectiverotation UNITED STATES PATENTS and the locking of the roller in aposition so that a desired tension may be applied to the screen. L5464()0 7/1925 Nichols 38/1015 2,8-2J7l 4/1958 Batey 38/102191 16 Claims, 9Drawing Figures Sheet 1 of4 Sept. 30,1975

US. Patent U.S. Patent Sept. 30,1975 Sheet 2 of4 3,908,293

US. Patent Sept. 30,1975 Sheet3 0f4 3,908,293

SCREEN TENSIONING AND PRINTING FRAME BACKGROUND This invention relatesto screen printing frames for supporting and stretching a printingscreen of flexible fabric. The devices disclosed in U.S. Pat. Nos.1,546,400; 3,601,912 and 3,482,343 are representative of the prior art.

A major disadvantage of the prior art such as the device in U.S. Pat.No. 1,546,400 is the inability to stretch the screen fabric uniformly.In said device, rollers engage the fabric at spaced points defined bypins.

In prior art devices exemplified by U.S. Pat. No. 3,482,343 the frame ischannel-shaped so as to receive a smaller cross-sectioned channel whichreciprocates by means of a plurality of screws. Devices as disclosed insaid patent are not sufficiently stable to do precision printing, suchas printed circuit boards, unless the frame is welded at the cornerjoints. Such frames lack versatility such that they can only be utilizedwith one size screen fabric, and cannot be disassembled for purposes ofstorage or shipment. Other deficiencies of devices as disclosed in saidpatent include an inherent limit to the extent of tensioning due to thelimited travel distance of the reciprocating bar. Deep channels arerequired for the frame in order to provide space for the reciprocatingbars, and thus the deep channel frame is inherently torsionallyunstable. In order to compensate for this instability, the channel frameis composed of exceedingly thick wall sections. This increases cost andweight.

The prior art devices as exemplified by U.S. Pat. No. 3,601,912, involveuse of rollers which constitute the tensioning mechanism and thestructural frame. The mechanical means for stopping rotation of theroller depends upon friction in an axial direction of the roller andhence the frame, under a sufficient load, can be caused to shift out ofits original plane. Also, the frame in said patent does not include anystructure or mechanism independent of the rollers for orientating theframe in a specific plane. The frame takes its orientation in the planesolely from the surface that the frame is lying on. Other disadvantagesof devices shown in said patent include the lack of any provision forproviding for an ink dam other than the roller itself, as well as theexcessive time and difficulty involved in the insertion of the stripthat locks the fabric to the roller.

The screen tensioning and printing frame of the present inventionincludes rollers interconnected at their ends by corner members. Eachroller is rotatably supported for rotation about its longitudinal axisby two adjacent corner members. Each roller has a shallow channel forreceiving and retaining an edge portion of a screen fabric.

In the frame of the present invention, a means is associated with eachcorner member for selectively rotating and locking the associated rollerso that a desired tension may be applied to the screen fabric. Further,a means is provided between adjacent corner members for applying acamber to the associated roller, and an ink dam means is provided on theframe inwardly of the rollers for sealing contact with the screenfabric.

Frames constructed in accordance with the present invention overcomevarious defects of the prior art while at the same time can be aninexpensive massproduced frame which will be perfectly square and flat.The frames of the present invention may be substantially lighter inweight than prior art frames such as those having channel-shaped framemembers.

The frame of the present invention has the advantage of continuoustake-up for tensioning of the screen fabric whereby the screen fabricmay be stretched to any desired tension for flat printing or contourprinting. The frame utilizes interchangeable components so that the sizeof theframe may be enlarged or diminished as desired, given a smallassortment of roller lengths. The rollers which are the structuralcomponent of the frame can be pre-stressed so as to develop an outwardcamber, whereby increased tension may be applied to the screen fabric,while the rollers still remain undeflected by the increased load appliedby the fabric. This maintains the parallel alignment of all of thefibers that reside in the same direction.

The locking coupling at the end of each roller assures that the frame isperfectly flat in the plane, as well as square. This registration in theplane is done independently of any external jigs or templates. Thesecouplings also assure slip free adjustment of the rollers. Assembly anduse of this frame was designed so as to require no special skills ortalents. Assembly of the frames with their pre-set flat alignment in theplane is accomplished within one or two minutes for any size frame.

Other objects and advantages will appear hereinafter,

For the purpose of illustrating the invention, there is shown in thedrawings a form which is presently preferred; it being understood,however, that this invention is not limited to the precise arrangementsand instrumentalities shown.

FIG, 1 is a perspective view ofa frame in accordance with the presentinvention having a screen fabric applied thereto.

FIG. 2 is a plan view of the frame shown in FIG. 1 but on an enlargedscale.

FIG. 3 is an enlarged detail view of a portion of FIG. 2.

FIG. 4 is a perspective exploded view of a corner member and roller endplug.

FIG. 5 is a sectional view taken along the line 55 in FIG. 3.

FIG. 6 is a view similar to FIG. 5 but showing another embodiment of thepresent invention with respect to the cross-section of the roller.

FIG. 7 is a perspective view of a hand tool for use in inserting theretainer.

FIG. 8 is an enlarged detail view of one corner of another embodiment ofthe present invention.

FIG. 9 is a sectional view taken along the line 99 in FIG. 8.

Referring to the drawing in detail, wherein like numerals indicate likeelements, there is shown in FIGS. 1 and 2 a screen tensioning andprinting frame 10 in accordance with the present invention. A screenfabric 12 is applied to one face of the frame 10 in a manner to bedescribed in detail hereinafter.

The frame 10 has its main structural element comprised ofa plurality ofrollers, namely rollers 14 and 18 parallel to each other and rollers 16and 20 which are parallel to each other. Roller 14 is mutuallyperpendicular with respect to rollers 16 and 20. The rollers 14-20 arerotatably supported at their ends by corner members 22, 24, 26 and 28.

The rollers 14-20 are preferably hollow rollers made of light weight,noncorrosive material such as aluminum. The corner members 22-28 arerigid members made from a light weight noncorrosive material such asaluminum. A pre-stressing means extends between adjacent corner membersfor applying a camber to the associated roller. As shown more clearly inFIGS. 2 and 3, such tension applying means includes a cable 30 havingthreads 32 at one end and a head 34 at the other end. Head 34 issufficiently large so that it cannot pass through hole 36 in a cornermember. A nut 38 is provided with an opposite corner member forcooperation with threads 32. Each head 34 is nonrotatable with respectto its associated hole 36 whereby rotation of nut 38 will apply a camberto the associated roller whereby the center portion of the roller willbe bowed outwardly.

Each corner member is provided with notches on mutually perpendicularfaces for receiving a dam member 40. Each dam member 40 is removablyreceived in oppositely disposed notches on the corner members 22-28. Thelowermost surface of each dam member is in sealing contact with a majorface of the screen fabric 12. In a preferred embodiment, the sealingcontact is preferably attained by means of an inflatable or hollow seal42 made from an elastomeric or polymeric plastic material. As shown inFIG. 5, seal 42 is removably retained on a lower surface of the dammember 40 by way of a notch.

All of the corner members 22-28 are identical. Hence, only corner member24 will be described in detail. Corner member24 has legs 44 and 46 whichare mutually perpendicular. Each leg contains a hole 36 through whichcable 30 may extend. Also, each leg of the corner member 24 is providedwith a hole larger in diameter than the hole 36 for rotatably supportingone end of an adjacent roller. In this regard, leg 44 is provided with ahole 48 and leg 46 is provided with a hole 50.

The holes 48 and 50 are of the same diameter and are identical. Hole 50is provided with a counter sink 52 so as to define a ledge on which isfixedly secured a ring 54. Ring 54 has radially disposed serrations orteeth 55 which face outwardly. See FIGS. 3 and 4. The inner diameter ofring 54 is slightly larger than or the same as the diameter of hole 50.Hole 48 is similarly constructed and provided with a ring 54.

Each of the rollers 14-20 is identical. Hence, only roller 18 will bedescribed in detail. Referring to FIG. 3, it will be noted that theroller 18 is a hollow tube 56 closed at opposite ends by end plugs 58and 60. The end plugs 58 and 60 are identical. The end plugs 58 and 60are removably attached in a nonrotative manner to the ends of the tube56 by means of screws 64 or the like.

The end plug 60 includes an enlarged diameter portion 61 which is of thesame diameter as tube 56 and adjacent thereto there is provided aratchet gear 62 concentric with the tube 56. A ratchet pawl 66 ispivotably secured on an interface of the leg 44 for cooperation with theratchet gear 62. See FIG. 5.

Adjacent the ratchet gear 62, the end plug 60 has a reduced diameterportion 68 which extends through the hole 50 and on its end face hasradially disposed serrations or teeth 70. Teeth 70 are concentric withand radially inwardly from the teeth 55 on ring 54.

The end plug 60 terminates in a threaded portion 72 on which is mounteda nut 80. Between nut 80 and ring 54, there is provided a ring 74. Ring74 has an inner set of radially disposed serrations or teeth 78 whichmate with teeth 70. Ring 74 also has an outer set of radially disposedserrations or teeth 76 which mate with the teeth 55 on ring 54. See FIG.3. When the teeth on ring 74 are mated with the teeth 55 on ring 54 andthe teeth on portion 68, the roller 18 is prevented from rotating aboutits longitudinal axis and is thereby locked in a predeterminedorientation.

The end plug 58 is telescoped into the opposite end of tube 56 andremovably retained therein by a screw 64 or similar device. End plug 58has an enlarged diameter wrench head 86 engaging an end face of the tube56. End plug 58 on roller 18 is releasably locked to corner member 26 inthe same manner as described above. Head 86 is of hexagonal or othershape where it can cooperate with a wrench or similar tool to facilitaterotation of the roller 18 about its longitudinal axis. The portion 61 ofend plug 60 could be fashioned as a hex to cooperate with a wrenchinstead of wrench head 86 if so desired.

Each of the rollers 14-20 releasably retains one edge portion of thescreen fabric 12 by means of a shallow channel 88 defined by a flatbottom wall and flanges 85, 87 which project toward one another. Aflexible retainer of plastic, metal or the like overlies an edge portionof the screen fabric 12 in each channel 88 and retains the edge portionlocked to the associated roller. See FIG. 5. Flanges 85, 87 may be ofthe same size but preferably flange 87 is longer than flange 85.

The retainer 90 is preferably insertable into the channel 88 only at oneend of the channel. To facilitate rapid insertion of retainer 90, thereis illustrated in FIG. 7 a tool designated generally as 92. Tool 92includes a plow 94 adapted to move through the channel 88 together withan upstanding rib 95 to facilitate manual manipulation of the tool 92.The tool 92 is releasably attached to one end of the retainer 90 bymeans of a set screw 96 which is rotated by manipulation of the head 97.The tool 92 facilitates pulling the retainer 90 along the channel 88with ease beginning at one end of the channel 88. If desired. a springclip may be utilized to retain temporarily the screen fabric 12 in oneend of the channel 88 while the retainer 90 is being inserted from theopposite end of the channel.

In FIG. 6, there is shown a cross-sectional view similar to FIG. 5 butof another embodiment of the present invention which is identical withthat described above except as will be made clear hereinafter. Thus, theonly significant different between the embodiment in FIGS. 1-5 versusthe embodiment of FIG. 6 is the roller 18'. Roller 18 utilizes a tube 98which is triangular in crosssection instead of being round incross-section as described above in connection with tube 56. The endplugs associated with tube 98 are round but have a triangular shapeportion which telescopes into the tube 98. All other correspondingelements illustrated in FIG. 6 are provided with corresponding primednumerals.

In FIGS. 8 and 9, there is illustrated another embodiment of the presentinvention designated generally as 10'. The frame 10 is the same as theframe 10 except as will be made clear hereinafter. Each corner member 99on the frame 10 may be made from a rigid polymeric plastic material.Like corner members 22-28, some flexibility is present in the legs ofthe corner members 99 to facilitate the application of an outwardlydirected camber on the rollers 108 and 110. It

will be appreciated that the frame has four rollers similar to therollers 14-20 described above.

Each leg of the corner member 99 has a pair of ribs 101 which define achannel 100 therebetween. A dam member 102 extends between the adjacentchannels 100 on adjacent corner members. The length of the dam member102 is less than the distance between the adjacent corner members 99 aswill be apparent from FIG. 8. The dam members 102 perform the additionalfunction of accommodating the means to apply a camber to the rollers108, 110.

As shown more-clearly in FIG. 9, the dam members 102 are generallyrectangular in cross-section, are hollow, and include a threaded bore106. A screw 104 extends through a hole in each leg of the cornermembers 99 and is engaged with the threaded bore 106. Rotation of screws104 on opposite ends of a dam member 102 applies a camber to theadjacent roller parallel thereto. A seal 112, similar to seal 42, isprovided on the lower surface of the dam members 102 for sealing contactwith the screen fabric 12. The frame 10 is otherwise identical as thatdescribed above.

Member 102 and cable 30 both perform the same prestressing function.Rotation of the screw 104 and nut 38 both cause elongation in member 102and cable 30 respectively. Such elongation applies tensile forces tomember 102 and cable 30. For equilibrium of the system, the tensileforces cause corresponding compressive forces in their associatedrollers 108 and 14. The combination of tensile and compressive forcesapply bending moments to the angle legs causing them to rotate therebyintroducing an outward deflection in the rollers 108 and 14. All rollerson each frame operate in the same fashion. Screws 104 and nuts 38 areadjusted so that all rollers remain straight (undeflected) afterloading, thus counteracting the opposing forces applied by the tensionedfabric.

In each embodiment, there is provided a plurality of corner members eachof which rotatably supports one end of mutually perpendicular rollers. Ameans extends between adjacent corner members for applying a camber tothe roller supported by the adjacent corner members. A means is providedfor positively locking a roller in any particular rotative position bymeans of a locking portion such as teeth. Thus, maintenance of anyparticular tension of the screen fabric does not rely on friction orsolely on a pawl associated with a ratchet whereby the frame may bestored in a horizontal, vertical, or upside down position withoutdisturbing the tension of the screen fabric. Since rotation of therollers is attained by means of a wrench cooperating with the wrenchhead 86, use of a torsion wrench facilitates application of accurateuniform tension on the screen fabric 12.

Before adjusting a roller such as roller 18, nut 80 is backed off andring 74 is moved along the axis of rotation to separate teeth 76 and 78from their complementary teeth. Thereafter, wrench head 86 may berotated. Pawl 66 will retain the roller 18 in the desired position untilthe locking ring 74 is applied as shown in FIG. 3.

The teeth on the ratchet gear 62 are of the same pitch as the teeth 70,78. Teeth 76 and 55 have the same pitch. Teeth 76 and 55 can be of thesame pitch as the teeth on ratchet gear 62 and the teeth 70, and 78, butit is not necessary. The commonality of these respective pitches assuresthat ring 74 will always nonrotatably couple plug 60 to corner member 24at every position for ratchet gear 62 caused by the engagement of pawl66. One advance step on ratchet gear 62 will cause one advance of teeth70. The teeth 55, 70, 76 and 78 are preferably straight V-shaped teethbut other configurations are possible such as involute.

For the maximum stability of the frames and most precise orientation inits plane, the leading point of the teeth 70 should not contact the rootdiameter of the teeth 78 and vice versa. The same is true for matingteeth 76 and 55. This arrangement will cause the mating teeth to bearand wedge against their inclined sides, thus eliminating any relativemovement between rollers and corner members when under load. that wouldotherwise occur if clearance was allowed to remain between mating teeth.

Teeth 78 could be axially disposed on the inner periphery of ring 74with the complementary teeth 70 being axially disposed on the outerperipheral of portion 68. Further, teeth 76 could be on the outerperiphery of ring 74 with teeth 55 being on the inner periphery of ring54 which would then surround ring 74.

The frame of the present invention is readily assembled or disassembledfor purposes of minimum storage, ease of shipment, and ease ofadjustment. Substitution of longer or shorter rollers, dam members andtensioning members facilitates changing the size of the frame for usewith a larger or smaller screen fabric. Each dam member is adapted to besealed with respect to the screen fabric 12 so that ink or otherprinting medium applied to the upper surface of screen fabric 12 in FIG.5 will not flow directlyto components located outwardly of the dammembers. Thus, the dam members define the operative portion of thescreen fabric 12.

The means for applying a camber to the rollers offsets any tendency ofthe rollers tobow inwardly as higher tensions are applied to the screenfabric 12.

Each retainer strip 90 increases its retention effect on an edge portionof the screen fabric 12 as the tension in the screen fabric 12 increaseswhile at the same time retaining uniformly the entire length of the edgeportion of the screen fabric 12 within the shallow channels 88. Thechannels 88 are shallow to the extent that in most embodiments they areequal to or less than 0.0625 inches deep whereby they do not interferewith the closed chamber within the rollers. Hence, the rollers havemaximum bending strength so that they may perform their function as themajor structured component of a frame. At the same time, it will benoted that the rollers facilitate continuous take up so that the screenfabric 12 may be stretched to any desired tension. i

It is very desirable to have channels 88 which are shallow and haveflanges85, 87 for cooperation with the retaining strip 90 as describedabove. Otherwise, the main structural components of the frame, namelyrollers 14-20, would be altered. As the depth of channel 88 increases.the cross sectional shape of the rollers approaches the shape of thecross-section of the torsionally unstable frame members of US. Pat. No.3,482,343.

The dam members 40, 40 may be inexpensive replaceable slats of wood,plastic, aluminum, etc. Dam members 102 are preferably made from a lightweight noncorrosive material such as aluminum. Each dam member may besealed to the screen fabric 12 in a manner other than by use of theflexible seals 42, 42 and 112. Each dam member has its ends located ator adjacent the intersection of the legs on its corner members.

More dimensional stability is imparted to the frame when and if darnmembers 102 bottom out so that its teeth engage mating teeth on cornermember 99. Thus, a dimensionally stable frame which is flat withadjacent dam members being mutually perpendicular is attained.

The present invention is primarily useful in printing by the silk screenmethod. However, it should be readily apparent that the presentinvention may be utilized in any one of a wide variety of industrieswherein tensioning of a screen fabric,'woven or nonwoven, is desired inconjunction with the various features and advantages of the frame of thepresent invention.

The present invention may be embodied in other specific forms withoutdeparting from the spirit or essential attributes thereof and,accordingly, reference 1 should be made to the appended claims, ratherthan to the foregoing specification as indicating the scope of theinvention.

I claim:

1. Apparatus comprising a screen tensioning and printing frame, saidframe having a plurality of rollers coupled together by corner memberswhich support the rollers for rotation about their longitudinal axes,each roller having a longitudinally extending channel on its periphery,a retainer for each channel to retain an edge portion of a screen fabricin each channel, means associated with each corner member for lockingeach roller in a predetermined rotative position so that a desiredtension may be applied to a screen fabric, means for applying a camberto each roller, and a dam member associated with each roller, each dammember being inwardlyof its associated roller and extending betweenadjacent corner members so that the dam members and corner members forma dam for a printing medium to be applied through a screen fabric.

2. Apparatus in accordance with claim 1 wherein said means for lockingeach roller includes a ring having teeth engageable with teeth on ajuxtaposed surface of its associated corner member.

3. Apparatus in accordance with claim 2 wherein said ring teeth includefirst and second concentric sets of teeth on a side face of the ring.

4. Apparatus in accordance with claim 1 wherein said camber applyingmeans is an adjustable means extending between adjacent corner membersso as to be parallel to its associated roller and inwardly of itsassociated roller.

5. Apparatus in accordance with claim 4 wherein said adjustable meansfor applying a camber is associated with each dam member, each dammember being hollow at least in part for accommodating an element toeffect such adjustment of the camber of a roller associated with eachdam member.

6. Apparatus in accordance with claim 1 including a flexible sealcarried by each darn member on a surface thereof for sealing contactwith a screen fabric.

7. Apparatus comprising a tensioning frame having a plurality of rollerscoupled together by corner members roller, each of said elements havinga locking portion which engages complementary locking portions on itsassociated roller and corner member, said locking elements being rings,each ring having teeth which constitute the locking portion of the ring,the complementary locking portions on said corner members and rollersincluding teeth for cooperative engagement with the teeth on saidlocking rings.

8. Apparatus in accordance with claim 7 wherein said teeth on said ringsinclude concentric sets of teeth on a side face thereof,

9. Apparatus in accordance with claim 7 including means extendingparallel to each roller between adjacent corner members for applying acamber to each roller.

10. Apparatus in accordance with claim 7 including a dam member inwardlyof each roller, each corner member including mutually perpendicularlegs, each dam member having an end disposed at or adjacent theintersection of the legs on each corner member associated therewith.

11. Apparatus in accordance with claim 7 wherein said fabric retainingmeans includes a shallow longitudinally extending channel havingoppositely disposed flanges at the mouth of the channel for retainingtherein a retainer strip which is wider than the distance between theflanges.

12. Apparatus in accordance with claim 7 wherein said locking elementsare sufficient in number so that each end of each roller is selectivelylocked to its associated corner member.

13. Apparatus comprising a dimensionally stable screen tensioning andprinting frame, said frame having a plurality of dam members coupledtogether by corner members, a roller generally parallel to each dammember, the rollers being supported forrotation about their longitudinalaxes by said corner members, each roller having a longitudinallyextending channel on its periphcry, a retainer foreach channel to retainan edge portion of a screen fabric in each channel, means associatedwith each corner member for locking each roller in a predeterminedrotative position so that a desired tension may be applied to a screenfabric, each dam member being inwardly of its associated roller andextending between adjacent corner members so that the dam members andcorner members form a dam for a printing medium to be applied through ascreen fabric.

14. Apparatus for comprising a tensioning frame having a plurality ofrollers coupled together by corner members which support the rollers forrotation about their respective longitudinal axes, each roller having ameans on its periphery to releasably retain an edge portion of a layerof flexible fabric, means associated with each corner member for lockingeach roller in a predetermined rotative position so that a desiredtension may be applied to a fabric, said locking means including adiscrete element associated with at least one end of each roller, eachof said elements having a locking portion which engages complementarylocking portions on its associated roller and corner member, saidlocking elements being rings, each ring having teeth which constitutethe locking portion of the ring, the complementary locking portions onsaid corner members and rollers including teeth for cooperativeengagement with the teeth on said locking rings, and each roller beingprovided with a plurality of discrete faces so as to be non-circularcross-sectioned.

15. Apparatus comprising a tensioning frame having a plurality ofrollers coupled together by corner members which support the rollers forrotation about their respective longitudinal axes, each roller having ameans on its periphery to releasably retain an edge portion of a layerof flexible fabric, means associated with each corner member for lockingeach roller in a predetermined rotative position so that a desiredtension may be applied to a fabric, said locking means including adiscrete element associated with at least one end of each roller, eachof said elements having a locking portion which engages complementarylocking portions on its associated roller and corner member, each rollerbeing a hollow cylindrical tube, an end plug fixedly secured to andclosing each of the tubes, each end plug cooperating with a separate oneof the corner members to facilitate rotation of its associated tubeabout the longitudinal axis thereof, and said locking means cooperatingwith at least one end plug of each roller and the associated member.

16. Apparatus in accordance with claim 1 wherein said channels areshallow, each channel having oppositely disposed flanges at the mouth ofthe channel for retaining therein the retainer associated therewith,each retainer being wider than the distance between the flanges of itschannel.

1. Apparatus comprising a screen tensioning and printing frame, saidframe having a plurality of rollers coupled together by corner memberswhich support the rollers for rotation about their longitudinal axes,each roller having a longitudinally extending channel on its periphery,a retainer for each channel to retain an edge portion of a screen fabricin each channel, means associated with each corner member for lockingeach roller in a predetermined rotative position so that a desiredtension may be applied to a screen fabric, means for applying a camberto each roller, and a dam member associated with each roller, each dammember being inwardly of its associated roller and extending betweenadjacent corner members so that the dam members and corner members forma dam for a printing medium to be applied through a screen fabric. 2.Apparatus in accordance with claim 1 wherein said means for locking eachroller includes a ring having teeth engageable with teeth on ajuxtaposed surface of its associated corner member.
 3. Apparatus inaccordance with claim 2 wherein said ring teeth include first and secondconcentric sets of teeth on a side face of the ring.
 4. Apparatus inaccordance with claim 1 wherein said camber aPplying means is anadjustable means extending between adjacent corner members so as to beparallel to its associated roller and inwardly of its associated roller.5. Apparatus in accordance with claim 4 wherein said adjustable meansfor applying a camber is associated with each dam member, each dammember being hollow at least in part for accommodating an element toeffect such adjustment of the camber of a roller associated with eachdam member.
 6. Apparatus in accordance with claim 1 including a flexibleseal carried by each dam member on a surface thereof for sealing contactwith a screen fabric.
 7. Apparatus comprising a tensioning frame havinga plurality of rollers coupled together by corner members which supportthe rollers for rotation about their respective longitudinal axes, eachroller having a means on its periphery to releasably retain an edgeportion of a layer of flexible fabric, means associated with each cornermember for locking each roller in a predetermined rotative position sothat a desired tension may be applied to a fabric, said locking meansincluding a discrete element associated with at least one end of eachroller, each of said elements having a locking portion which engagescomplementary locking portions on its associated roller and cornermember, said locking elements being rings, each ring having teeth whichconstitute the locking portion of the ring, the complementary lockingportions on said corner members and rollers including teeth forcooperative engagement with the teeth on said locking rings. 8.Apparatus in accordance with claim 7 wherein said teeth on said ringsinclude concentric sets of teeth on a side face thereof.
 9. Apparatus inaccordance with claim 7 including means extending parallel to eachroller between adjacent corner members for applying a camber to eachroller.
 10. Apparatus in accordance with claim 7 including a dam memberinwardly of each roller, each corner member including mutuallyperpendicular legs, each dam member having an end disposed at oradjacent the intersection of the legs on each corner member associatedtherewith.
 11. Apparatus in accordance with claim 7 wherein said fabricretaining means includes a shallow longitudinally extending channelhaving oppositely disposed flanges at the mouth of the channel forretaining therein a retainer strip which is wider than the distancebetween the flanges.
 12. Apparatus in accordance with claim 7 whereinsaid locking elements are sufficient in number so that each end of eachroller is selectively locked to its associated corner member. 13.Apparatus comprising a dimensionally stable screen tensioning andprinting frame, said frame having a plurality of dam members coupledtogether by corner members, a roller generally parallel to each dammember, the rollers being supported for rotation about theirlongitudinal axes by said corner members, each roller having alongitudinally extending channel on its periphery, a retainer foreachchannel to retain an edge portion of a screen fabric in each channel,means associated with each corner member for locking each roller in apredetermined rotative position so that a desired tension may be appliedto a screen fabric, each dam member being inwardly of its associatedroller and extending between adjacent corner members so that the dammembers and corner members form a dam for a printing medium to beapplied through a screen fabric.
 14. Apparatus for comprising atensioning frame having a plurality of rollers coupled together bycorner members which support the rollers for rotation about theirrespective longitudinal axes, each roller having a means on itsperiphery to releasably retain an edge portion of a layer of flexiblefabric, means associated with each corner member for locking each rollerin a predetermined rotative position so that a desired tension may beapplied to a fabric, said locking means including a discrete elementassociated with at least one end of each roller, each of said Elementshaving a locking portion which engages complementary locking portions onits associated roller and corner member, said locking elements beingrings, each ring having teeth which constitute the locking portion ofthe ring, the complementary locking portions on said corner members androllers including teeth for cooperative engagement with the teeth onsaid locking rings, and each roller being provided with a plurality ofdiscrete faces so as to be non-circular cross-sectioned.
 15. Apparatuscomprising a tensioning frame having a plurality of rollers coupledtogether by corner members which support the rollers for rotation abouttheir respective longitudinal axes, each roller having a means on itsperiphery to releasably retain an edge portion of a layer of flexiblefabric, means associated with each corner member for locking each rollerin a predetermined rotative position so that a desired tension may beapplied to a fabric, said locking means including a discrete elementassociated with at least one end of each roller, each of said elementshaving a locking portion which engages complementary locking portions onits associated roller and corner member, each roller being a hollowcylindrical tube, an end plug fixedly secured to and closing each of thetubes, each end plug cooperating with a separate one of the cornermembers to facilitate rotation of its associated tube about thelongitudinal axis thereof, and said locking means cooperating with atleast one end plug of each roller and the associated member. 16.Apparatus in accordance with claim 1 wherein said channels are shallow,each channel having oppositely disposed flanges at the mouth of thechannel for retaining therein the retainer associated therewith, eachretainer being wider than the distance between the flanges of itschannel.